Nearly all practical transceivers require some form of filtering. Up to the date, in majority of cases, these radio-frequency (RF) and intermediate-frequency (IF) filters are realized off-chip as ceramic or surface-acoustic wave (SAW) devices.
A first reason for slow progress in integration RF and IF filters is their rather modest noise and distortion performance. This can be alleviated by a careful overall system design by taking into account the filter short comings and by purposely reducing their requirements while simultaneously offsetting their reduced performance with superior performance of preceding and following high-quality blocks.
A second reason for slow progress in integration RF and IF filters is that these filters require circuitry for adjusting their center or corner frequency as well as their quality-, or (Q)-factors. The accuracy problems of such tuning systems may result in the whole filter not meeting the stringent system specifications over process, voltage supply and temperature variation (PVT). Due to the matching errors the most frequently used Master-Salve (M-S) schemes suffer from significant accuracy errors averaging up to 5% for frequency schemes and up to several tens of percent for Q-tuning schemes. Naturally, for majority radio applications such a modest accuracy is not acceptable. Additionally, the reference feed-through degrades the overall noise performance of the filter. The typically achievable S/N ratio of an active filter tuned with a M-S scheme is about 40 dB.
A third reason for slow progress in integration RF and IF filters is not taking full advantage of possible system and circuit trade-offs during the transceiver design. In order to make a design of RF or IF filters viable, their system specification should be optimized and carefully negotiated with the overall system specifications. In other words, for a successful implementation of a fully-integrated transceiver the sequence of specification building should be reversed: knowing the limitations of the active filters one should design the system architecture, its system specifications and other circuits to alleviate these shortcomings. Only then the whole system has a chance to meet its overall requirements.
In the case of the presented RF and IF filters the tuning accuracy can be substantially improved with tuning the filter signal directly instead of using the Master-Slave (M-S) scheme by passing the reference through it while simultaneously processing the signal. The expected accuracy of such frequency- and Q-tuning systems could reach 0.5% and 2% respectively. There are certain requirements for the reference signal that need to be fulfilled:
its frequency should fall at the edge of the pass-band of the RF filter, but its frequency should be chosen so that it will not inter-modulate with the adjacent channel carrier;
the reference amplitude of the RF filter should be at least 15 dB lower than the selected channel carrier;
given these conditions the reference passes the RF band-pass (BP) filter linearly with causing inter-modulation distortion. Suppose that the reference frequency it is Δf away from the desired channel carrier. Give the attenuation characteristic of the IF BP filter Δf separation should be chosen such that the IF BP filter attenuates the reference of the RF BP filter by at least 40 dB;
hence at the output the signal and the reference differ at least by 55 dB, which is better than the reference feed-through of a typical M-S scheme;
the reference of the IF BP filter is rejected by its appropriate conditioning with respect to the main signal and the complex nature of the IF BP filter. The expected attenuation of the IF BP filter reference is at least 55 dB.